Histamine is a well established modulator of neuronal activity. At least four subtypes of histamine receptors have been reported in the literature—H1, H2, H3, H4. The histamine H3 receptors play a key role in neurotransmission in the central nervous system. The H3 receptor was discovered in 1983 originally on histamine-containing neurons where it was shown to function presynaptically, regulating the release and synthesis of the biogenic amine histamine (Arrang et al, 1983) now a well established neurotransmitter. H3 receptors are predominately expressed in the brain, localizing to the cerebral cortex, amygdala, hippocampus, striatum, thalamus and hypothalamus. H3 receptors are also localized presynaptically on histaminergic nerve terminals and act as inhibitory autoreceptors (Alguacil and Perez-Garcia, 2003; Passani et al, 2004; Leurs at al, 2005; Celanire et al, 2005; Witkin and Nelson, 2004). When these receptors are activated by histamine, histamine release is inhibited. H3 receptors can also be found in the periphery (skin, lung, cardiovascular system, intestine, GI tract, etc). H3 receptors are also involved in presynaptic regulation of the release of acetylcholine, dopamine, GABA, glutamate and serotonin (see Repka-Ramirez, 2003; Chazot and Hann, 2001; Leurs et al, 1998). The H3 receptor demonstrates a high degree of constitutive or spontaneous activity (e.g., receptor is active in the absence of agonist stimulation) in vitro and in vivo, thus, ligands to the receptor can display, agonist, neutral antagonist or inverse agonist effects.
The location and function of histaminergic neurons in the CNS suggests that compounds interacting with the H3 receptor may have utility in a number of therapeutic applications including narcolepsy or sleep/wake disorders, feeding behavior, eating disorders, obesity, cognition, arousal, memory, mood disorders, mood attention alteration, attention deficit hyperactivity disorder (ADHD), Alzheimer's disease/dementia, schizophrenia, pain, stress, migraine, motion sickness, depression, psychiatric disorders and epilepsy (Leers et al, 2005; Witkin and Nelson, 2004, Hancock and Fox 2004; Esbenshade et al. 2006). An H3 antagonist/inverse agonist could be important for gastrointestinal disorders, respiratory disorders such as asthma, inflammation, and myocardial infarction.
Ohtake et al. (US 2006/0178375 A1) disclosed compounds that reportedly exhibit histamine receptor H3 antagonist or inverse agonist activity and may be useful for the treatment or prevention of obesity, diabetes, hormonal secretion abnormality, or sleep disorders.
Celanire et al. (WO 2006/103057 A1 and WO 2006/103045) have disclosed compounds comprising an oxazoline or thiazoline moiety, processes for preparing them, their pharmaceutical compositions and their uses as H3 ligands.
Bertrand et al. (WO 2006/117609 A2) disclosed novel histamine H3 receptor ligands, processes for their preparation, and their therapeutic applications.
Schwartz et al. (WO 2006/103546 A2) disclosed certain methods of treatment for Parkinson's disease, obstructive sleep apnea, narcolepsy, dementia with Lewy bodies, and/or vascular dementia using non-imidazole alkylamine derivatives that are antagonists of the H3 receptors of histamine.
Apodaca et al. (EP 1 311 482 B1) disclosed certain non-imidazole aryloxypiperidines as H3 receptor ligands, their synthesis, and their use for the treatment of disorders and conditions mediated by the histamine receptor.
Xu et al. disclosed certain 6-substituted phenyl-4,5-dihydro-3(2H)-pyridazinones, their synthesis, and rabbit platelet aggregation inhibitory activity induced by ADP in vitro.
Thus, there is a need for novel classes of compounds that possess the beneficial properties. It has been discovered that currently disclosed class of compounds, referred to herein as substituted pyridazinone derivatives, are useful as agents for treating or preventing various diseases or disorders disclosed herein.